Field of the Invention
The present invention relates to purification of biogas, in particular, of digester gas or landfill gas using an integrated gas separation system including TSA and membranes.
Related Art
Biogas contains impurities of H2S, volatile organic compounds (VOCs), water, CO2 and air. Removal of such impurities can yield nearly pure methane for sale as natural gas. Two typical types of biogas are landfill gas and digester gas. Landfill gas is obtained from a landfill where microorganisms convert waste primarily to methane and CO2. Digesters anaerobically ferment agricultural, human waste, or other organic containing sources also resulting primarily in methane and CO2. While the main constituents of biogas are methane and CO2, they also include minor levels of water vapor, VOCs, CO2, H2S, and sometimes siloxanes (i.e., in landfill gas). The H2S present in biogas, especially for high H2S levels often found in digester gas, poses an economic and technical challenge because the product natural gas must meet requirements of relatively low H2S levels in order for it to be useful as a fuel or meet pipeline specifications. For example, while natural gas pipelines typically require an H2S level of 4 ppm (v/v) or less and a CO2 level of 2% (v/v) or less, digester gas often contains CO2 levels of 25-45% (v/v) and relatively high H2S levels of 100 ppm-1% (v/v).
Many technologies today are applied to upgrade digester gas including a patented (U.S. Pat. No. 7,025,803) purification system offered by Air Liquide Advanced Technologies, US. This system includes a pressure swing adsorption (PSA) unit followed by an activated carbon bed for water and VOC removal. The water and VOC-depleted gas is then fed to a two stage gas separation membrane unit for removal of CO2. The first stage removes the bulk of the CO2 present in the biogas. The CO2 rich reject stream at low pressure from the first stage is used to regenerate the PSA unit to produce an impurity-laden CO2 rich reject stream containing methane (lost from the feed gas via permeation through the membranes of the first stage), rejected CO2 and desorbed VOCs and water. This stream is typically routed to a thermal oxidizer for destruction of the VOCs prior to venting. The methane rich second stage permeate is also at low pressure, so it is recycled to the suction inlet of the compressor upstream of the PSA unit. While this system has performed remarkably well, it does not satisfactorily handle relatively high levels of H2S, and for that reason, a separate H2S removal system (such as SulfaTreat or other treatment method) may required for raw biogas containing relatively high H2S levels. Inclusion of the separate H2S removal system adds cost and complexity to the overall system.
A key advantage of the above-described Air Liquide system for treatment of biogas from landfills is removal by the membrane unit of a bulk of the O2 in the biogas along with the CO2. While pipeline specifications for O2 may vary, a typical requirement is O2 levels no higher than 0.2% (v/v). While biogas obtained from digesters should be O2 free due to the anaerobic conditions of the digesters, digesters are low pressure operations that may allow of introduction of some amounts of air. Thus, some amount of O2 is commonly encountered in digester-derived biogas.
Another digester gas upgrading system is offered by Guild Associates, Inc. One Guild system includes a PSA system that has the ability to adsorb H2O, H2S and CO2 in a single unit. A key attribute of this system is its ability to simultaneously adsorb and desorb H2S, water and CO2. However, this technology is limited for feeds containing O2 and N2 since the PSA unit enriches O2 and N2 in the product gas, typically by a factor of about 1.7× the feed gas concentration. Thus, for a product containing 2000 ppm O2 (v/v), the raw gas fed to the PSA unit is limited to an O2 level of only 1200 ppm (v/v). If the O2 limit of the raw gas is exceeded, an additional process unit or units for removal of O2 will be required. Inclusion of an additional process unit or units for removal of O2 adds cost and complexity to the overall system.
Other PSA systems for digester gas upgrading have been proposed. However, many of such systems typically include a pretreatment system for H2S removal, thus adding cost and complexity.
Water-wash systems have been proposed for upgrading digester gas. Such systems include an air-stripped stream of water that is contacted over a packed bed against a rising feed stream. CO2 present in the feed stream dissolves into the stream of water. The CO2-laden water stream is subsequently let down in pressure and stripped with air for removal of the dissolved CO2 derived from the feed stream. In water-wash system, amounts of H2S present in the feed stream may also be removed through dissolution in the water stream. In such a case, the regenerated stream is the stripping air plus the CO2 and H2S. As with the Guild system, O2 and N2 are not removed but instead are enriched in the product stream.
Similarly to the water-wash system, amine or physical solvent based upgrading systems have also been proposed for upgrading of biogas. In such systems, the solvent absorbs CO2 and H2S present in the feed stream, and after pressure letdown, the solvent is regenerated by reboiling the solvent to drive off the previously absorbed CO2 and H2S. In other words, external stripping air is not used. For physical solvents, the reboiling can be reduced or in some cases eliminated and pressure letdown of the rich solvent may be used alone for regeneration. However, similar to the Guild system and water-wash systems, amine or physical solvent based systems enrich O2 and N2 in the product stream.
Thus, is an object to upgrade biogas, particularly digester gas, that includes O2 and relatively high levels of H2S using a system that does not have an unnecessarily high cost or level of complexity.